1. Field of the Invention
The present invention relates to a tunable filter preferably for use as a band filter in a communication system and in which a frequency band can be adjusted, and more particularly, to a tunable filter including a surface acoustic wave resonator.
2. Description of the Related Art
Occasionally, a bandpass filter used in a communication system having an adjustable pass band is required. Various bandpass filters, i.e., tunable filters, that satisfy this requirement have been suggested.
For example, Japanese Unexamined Patent Application Publication No. 2005-217852 discloses a tunable filter that uses a plurality of surface acoustic wave resonators and a plurality of variable capacitors. FIG. 17 is a circuit diagram of the tunable filter described in Japanese Unexamined Patent Application Publication No. 2005-217852.
In a tunable filter 101, a plurality of series-arm resonators 104 and 105 are connected in series with a series arm that couples an input terminal 102 and an output terminal 103 to each other. In addition, parallel-arm resonators 106 and 107 are respectively connected with a plurality of parallel arms provided between the series arm and a ground potential. The series-arm resonators 104 and 105 and the parallel-arm resonators 106 and 107 are defined by surface acoustic wave resonators.
A ladder filter circuit including the series-arm resonators 104 and 105 and the parallel-arm resonators 106 and 107 is provided. Further, to allow the pass band to be adjusted, variable capacitors 108 to 115 are provided. In particular, the variable capacitor 108 is connected in parallel with the series-arm resonator 104, and the variable capacitor 110 is connected in series with the series-arm resonator 104 and the variable capacitor 108. Similarly, the variable capacitor 109 is connected in parallel with the series-arm resonator 105, and is connected in series with the variable capacitor 111.
The variable capacitor 112 is connected in parallel with the parallel-arm resonator 106, and the variable capacitor 114 is connected in series with the parallel-arm resonator 106 and the variable capacitor 112. Similarly, the variable capacitor 113 is connected in parallel with the parallel-arm resonator 107, and is connected in series with the variable capacitor 115.
In the tunable filter 101, a resonant frequency FrS of a circuit portion of the series arm is increased as the capacities of the variable capacitors 110 and 111, i.e., the series capacity is decreased. Also, an anti-resonant frequency FaS of the series arm is decreased as the parallel capacity, i.e., the electrostatic capacities of the variable capacitors 108 and 109 are increased.
Similarly, a resonant frequency FrP and an anti-resonant frequency FaP of a circuit portion of the parallel arms can be changed by changing the capacities of the variable capacitors 112 and 113 that are connected in parallel and the capacities of the variable capacitors 114 and 115 that are connected in series. Accordingly, the center frequency of the entire tunable filter 101 can be changed by changing the capacities of the variable capacitors 108 to 115.
However, with the tunable filter 101 described in Japanese Unexamined Patent Application Publication No. 2005-217852, the surface acoustic wave resonators used for the series-arm resonators 104 and 105 and the parallel-arm resonators 106 and 107 have small electromechanical coupling coefficients. Also, a sufficient band width ratio or a sufficient variable amount has not been obtained. Further, there has been a problem in which an absolute value of a temperature coefficient of frequency TCF is large.